I have a 50mm 1.4, what would be the advantage of the 40mm? Does it produce better images, or would it just be slightly wider?
In my opinion the 40 2.8 should not be rated after it's aperture as 2.8 isn't very special for this kind of focal length but it's a perfect lens to have for absolute light weight and small size requirements. Like when I was skiing in early 2013 I almost only carried my 5D3 with the 40 2.8 in my backpack on the hill because it takes no more space than the body itself and delivers an outstanding image quality for its size and price. I admit it looks a bit unmanly though.
If you're not that into having this for a fun purpose or for when you want the lightest combination possible I would not get it, but it's really worth a try if you do.